Traditionally, in separating air, if argon is to be obtained as a product gas, the incoming air is separated into relatively pure streams of oxygen, nitrogen and argon. The ideal thermodynamic work involved in such a separation is 14.5 KCal/SM.sup.3. Since air contains less that 1% by volume of argon, this traditional `total-split` air separation technology is particularly inefficient if argon is the only desired product. The ideal thermodynamic work of a process for separating air into an argon stream and an oxygen-nitrogen mixture is only 1.2 KCal/SM.sup.3.
In order to improve the efficiency of argon recovery, we believe that it is desirable to separate air into oxygen, nitrogen and argon in a conventional distillation system operating at cryogenic temperatures, but to remix the oxygen and nitrogen so as to recover the work of mixing typically in the form of heat pump duty for the distillation system. We have found that the overall efficiency in terms of argon production of such a process is highly dependent upon the efficiency with which the mixing is performed.
European patent application No. 136 926A relates to the operation of a conventional double column with argon "side-draw" for producing nitrogen, oxygen and argon products. It is the object of the invention disclosed in that European patent application to take advantage of a temporary fall in the oxygen demand in order to increase one or more of the other products, for example argon. A liquid is thus taken from one of the two columns forming the double column and is passed to the top of an auxiliary or mixing column operating at substantially the pressure of the low pressure column. A gas whose oxygen content is less than that of the liquid that is taken from the low pressure column is passed to the bottom of the auxiliary column. The liquid collected at the bottom of the auxiliary column is passed as reflux into the low pressure column substantially at the level from where the said gas is taken. As more oxygen-rich liquid is taken from the double column and passed to the auxiliary column so more reflux may be provided for the low pressure column, thereby making possible an increase in the rate of argon production. However, this method involves substantial inefficiencies which makes it unsuitable for use in a plant for producing argon as the primary or sole product of air separation. In particular, the only heat extracted from the top of the column is that in a waste stream comprising oxygen and nitrogen that is vented from the top of the mixing or auxiliary column. In addition the amount of liquid oxygen that can be added to the top of the column is restricted by the need for a mass balance with the oxygen vented in the waste stream. Accordingly, the amount of heat pumping duty that can be performed is limited. Moreover, by rejecting the waste stream comprising oxygen and nitrogen from the top of the column it is inevitable that at least in some parts of the column the operating conditions will diverge substantially from equilibrium conditions with a concomitant loss of thermodynamic efficiency. If the liquid introduced into the top of the mixing column is pure oxygen, the divergence will be particularly marked, while if the liquid contains argon there will also be an appreciable fall in the argon yield from the plant.
It is an aim of the invention to provide an improved method and apparatus for separating argon from a gas mixture comprising argon, nitrogen and oxygen.